1. Field of the Invention
The invention is directed towards a method and apparatus for controlling the output current of a multiple output transconductance system, and in particular to a method and apparatus for controlling a microelectromechanical system (MEMS) device such as a steerable mirror.
2. Description of the Related Art
MEMs devices and systems have proven useful in numerous sensing and actuating applications. Illustrative applications for MEMS devices include optical switches, inertial or pressure sensors, and biomedical devices. MEMS devices can also be used in a switching capacity in the telecommunication industry, particularly in optical telecommunication systems.
In optical telecommunication systems, MEMS devices are called on to switch the path of the transmitted light, sometimes referred to as beam steering. In beam steering, the light from the fiber is selectively deflected or steered by one or more movable optical element from an input fiber to an output fiber. Suitable optical elements for performing this task include (MEMS) mirrors.
MEMS may be fabricated on semiconductor substrates, typically silicon substrates. These microelectromechanical systems typically have sizes on the order of microns and may be integrated with other electrical circuits on a common substrate. Present MEMS-based optical switches can operate in the plane of the substrate or normal to the substrate. Because each fiber in an optical telecommunication system has a small “acceptance window”, some degree of accuracy is required in positioning the mirror to direct the transmitted light to the output fiber.
The actuation force used to move MEMS mirrors in an optical cross-connect system is typically electrostatic, electromagnetic, piezoelectric or thermal. As part of the mirror control system, a feedback circuit is generally used to monitor the position of the mirror. In electrostatically actuated mirror applications, generally high voltages (on the order of 80–200 volts) may be required to position the mirror. Typically, however, dealing with such high voltages in the design of multiple mirror systems (having, for example, hundreds or thousands of mirrors on a single application) presents a number of difficulties. Although the high voltages are required to position the mirror, designers generally desire to use lower voltages to control the mirrors and other aspects of the circuits in the mirror array to save heat and power.
FIG. 1 illustrates a generalized block level diagram of a prior art configuration for positioning a MEMS mirror 100 using capacitive electrostatic coupling. Two control pads 102, 104 are positioned relative to a mirror surface 100. The mirror 100 may be fabricated in accordance with any of a number of well known MEMS fabrication techniques out of a semiconductor material which is polished and positioned in accordance with well known techniques as reflected in: “Embedded interconnect and electrical isolation for high-aspect-ratio, SOI inertial instruments”, Brosnihan, T. J.; Bustillo, J. M.; Pisano, A. P.; Howe, R. T., Solid State Sensors and Actuators, 1997. TRANSDUCERS '97 Chicago, 1997 International Conference, Volume: 1, 1997; “Single-chip surface-micromachined integrated gyroscope with 50/spl deg//hour root allan variance”, Geen, J. A.; Sherman, S. J.; Chang, J. F.; Lewis, S. R., Solid-State Circuits Conference, 2002. Digest of Technical Papers. 2002 IEEE International, Volume: 2, 2002, Page(s): 346–539. A voltage between the force pad 102, 104 and the mirror 100 generates an electrostatic force to move the mirror relative to the pad. While only two pads 102, 104 are illustrated, many systems use four or more pads for true three dimensional movement.
The mirror is positioned relative to the substrate and pads 102, 104 using voltage generated by an amplifier 110 coupled to control pads adjacent to the mirror to be controlled. In FIG. 1, a high voltage amplifier 110 (generally on the order of 80–200 v) such as that described in co-pending U.S. patent application Ser. No. 09/944,930 entitled “High Voltage Integrated Current Amplifier”, by inventor Mark Lemkin, commonly assigned, is used to generate voltages sufficient to provide electrostatic actuation between the control pads 102, 104 and the mirror 100. A low voltage transconductance circuit may be optionally provided in place of the amplifier to convert the voltage to a current to drive the control pads.
As noted above, in optical switching applications, one needs to understand the position of the mirror. In order to determine the position of the mirror, a common technique is to provide a position sense channel 120 coupled to the mirror. Generally, the sense channel 120 detects the voltage present at the interface between the mirror and the control pad, and through a series of measurements, position of the mirror is determined.
Normally the sense channel 120 will be designed using low voltage components which results in a number of integration issues with the high voltage devices used to position the mirror. Any position sense channel design must deal with the fundamental issues of providing feedback from the output of the system (a high voltage amplifier output) without using a DC current or a high voltage switch. Use of a DC current feedback system is impractical in high density switch applications as too much power would be required on the circuit. High voltage switches are generally costly in terms of the amount of size required to implement them.